U.S. public health officials must prepare now for the inevitable arrival of Zika virus, a mosquito-borne infection that has spread to 22 countries and territories in the Americas and poses particular danger to pregnant women, health experts said.

The virus is expected to spread to the United States and every country in the Western hemisphere where Aedes mosquitoes, which spread the virus, are known to live, according to the Pan American Health Organization. Aedes mosquitoes live in every Western hemisphere country but Canada and Chile.

Zika poses little risk to most people. But health officials are worried about its link to Brazil's recent increase in a type of birth defect called microcephaly, in which infants are born with small skulls and incomplete brain development, said Lawrence Gostin, director of the O'Neill Institute for National and Global Health Law at Georgetown University in Washington.

International air travel will help the virus spread quickly, said Lawrence Gostin, director of the O’Neill Institute for National and Global Health Law at Georgetown University in Washington. Zika doesn’t spread from person to person, but a mosquito carrying the virus could hitch a ride on the plane and end up in the USA. An American mosquito could become a carrier of the virus if it bites an infected person who contracted the virus while traveling in an affected country.

Zika "will certainly come to the United States, and I think it will come fairly rapidly," Gostin said.

Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said the USA could see a small cluster of Zika cases, but would likely be able to contain the outbreak, just as it has contained recent outbreaks of dengue fever and chikungunya, tropical diseases spread by the same mosquito that carries Zika.

"We’ve been able to put the lid on those mild outbreaks, with mosquito control and elimination," Fauci said.

The Centers for Disease Control and Prevention issued a travel alert earlier this month, warning pregnant women to avoid travel to areas where Zika is spreading. Most recently, the CDC added the Dominican Republic and the U.S. Virgin Islands to this list. Researchers in Brazil and at the National Institute of Health are working on a Zika vaccine. There are no approved treatments.

President Obama met Tuesday with his health and national security teams to discuss Zika's spread and its potential economic and developmental impact.

"The president emphasized the need to accelerate research efforts to make available better diagnostic tests, to develop vaccines and therapeutics, and to ensure that all Americans have information about the Zika virus and steps they can take to better protect themselves from infection," the White House said.

To prepare for the Zika, U.S. public health officials should look for the virus both in mosquitoes and in travelers returning from outbreak areas, Gostin said.

"We need to really be on the lookout," Gostin said. "That involves having really good information systems from the CDC. It means training doctors. The last thing we want is a repeat of Ebola, where we saw preventable deaths in the United States and globally."

Communities also need to ramp up their mosquito control efforts by spraying and getting rid of standing water, Gostin said.

Trash on the side of the road creates breeding grounds for Aedes mosquitoes, said Michael Osterholm, director of the University of Minnesota's Center for Infectious Disease Research and Policy. Trash is far more common in poor neighborhoods in developing nations than in wealthy ones like the USA.

"These mosquitoes have adapted very well to our throw-away society," Osterholm said. "It's not in the swamps where the mosquitoes that spread malaria live. But that discarded fast food wrapper in the ditch could be a very important source of Aedes."

The USA also could take the controversial step of releasing genetically engineered mosquitoes, Gostin said. Releasing sterile male Aedes mosquitoes, who would not be able to reproduce, could help reduce the mosquito population. Although some worry about the unknown effects of releasing genetically engineered mosquitoes into the environment, this approach could become an attractive option in an outbreak, he said.

Health officials should also test pregnant women, Gostin said. But such testing is complicated. There are no commercially available diagnostic tests, such as those for the flu or strep throat. Local health officials usually send blood samples to the Centers for Disease Control and Prevention in Atlanta. Only a handful of state labs can test for Zika, Gostin said.

"If we have a baby born, or many babies born, with malformations that could have been prevented, it will be unforgivable, he said.

The government of El Salvador has urged women to postpone pregnancies for two years, a move that has been viewed as extreme by most people in the public health community. About half of pregnancies in the United States are unplanned.

Scientists have known about Zika virus since the 1940s, but it was not considered a serious threat until a few months ago, when Brazil reported the increase in microcephaly.

Brazil's Health Ministry said Wednesday it had recorded 4,120 suspected cases of microcephaly since October, and have confirmed 270 through lab tests, the Associated Press reported. The ministry ruled out 462 and is still investigating 3,448 cases. The country usually has 100 to 200 microcephaly cases per year.

Scientists don't know why Zika is spreading so quickly or why the outbreak has been particularly bad in Brazil, said Christian Lindmeier, a spokesman for the World Health Organization (WHO). While Aedes mosquitoes clearly play a key role in Zika outbreaks, it's possible that a second, unknown factor has led the outbreak in Brazil to be especially large. Brazil has said that there have been 400,000 to 1.4 million Zika infections in that country.

WHO has not issued any travel advisories.

Although Brazil reported its first Zika cases in May, some researchers speculate that Zika actually arrived in Brazil during the World Cup soccer games in 2014, which brought people to Brazil from all over the world. Most people with Zika exhibit no symptoms so the virus can spread widely before it's noticed. When symptoms do occur, they are usually mild and include a low fever, headaches, joint pain and a rash, Lindmeier said.

Warming global temperatures, due to both climate change and the current El Niño climate pattern, could move mosquitoes to new terrain, taking Zika with them, said Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine in Houston.

The USA is also vulnerable to Zika in entrenched pockets of poverty, particularly around the Gulf Coast, Hotez said. Mosquito-borne diseases spread more quickly in poor neighborhoods because they often have more trash on the ground and homes there often lack window screens. People who don't have air conditioning also spend more time outside, where they're more likely to be bitten.

The CDC has confirmed Zika virus in about a dozen Americans in a handful of states who had traveled to Latin America, including a case in Minnesota confirmed Wednesday. There is no evidence Zika is spreading among mosquitoes native to the continental USA, although the virus does appear to be spreading in Puerto Rico and the U.S. Virgin Islands.

The USA has had sporadic cases of Zika virus in travelers in the past; none of these cases caused outbreaks. The CDC diagnosed 14 returning travelers with Zika from 2007 to 2014.

Some infectious disease experts doubt Zika will spread widely in the continental USA.

In Africa, Zika outbreaks have often followed on the heels of a virus called chikungunya, which also causes fever, rash and joint pain, and is also spread by Aedes mosquitoes. Chikungunya made its first appearance in the Western Hemisphere in 2013 and has since spread rapidly throughout the Caribbean, but has caused only a handful of cases in the continental USA, Osterholm said.

If Zika continues to follow this pattern, the virus may stop posing much of a threat to the continental USA, Osterholm said.

But many health officials worry that the Olympics, to be held in Rio de Janeiro this summer, could spread the virus around the world.

"The Olympics will be a perfect dissemination vehicle for Zika, as people from all over the world will converge in Brazil and then return to their home countries, which might also harbor Aedes mosquitoes," said Amesh Adalja, a senior associate at the Center for Health Security at the University of Pittsburgh Medical Center.